1. Field of the Invention
The present invention relates generally to soil remediation, and more particularly to a heater for an in situ thermal desorption soil remediation process.
2. Description of Related Art
Contamination of subsurface soils has become a matter of great concern in many locations. Subsurface soil may become contaminated with chemical, biological, and/or radioactive contaminants. Contamination of subsurface soil may occur in a variety of ways. Hazardous material spills, leaking storage vessels, and landfill seepage of improperly disposed of materials are just a few examples of the many ways in which soil may become contaminated. Contaminants in subsurface soil can become public health hazards if the contaminants migrate into aquifers, into air, or into the food supply. Contaminants in subsurface soil may migrate into the food supply through bioaccumulation in various species that are part of the food chain.
There are many methods for removal of contaminants from subsurface soil. Some possible methods for treating contaminated subsurface soil include excavation followed by incineration, in situ vitrification, biological treatment, and in situ chemical treatment. Although these methods may be successfully applied in some applications, the methods can be very expensive. The methods may not be practical if many tons of soil must be treated.
One process that may be used to remove contaminants from subsurface soil is a soil vapor extraction (SVE) process. A SVE process applies a vacuum to a well to draw air through subsurface soil. The air carries volatile contaminants towards the source of the vacuum. Off-gas removed from the soil by the vacuum may include contaminants that were within the soil. The off-gas may be transported to a treatment facility. The off-gas removed from the soil may be processed in the treatment facility to reduce contaminants within the off-gas to acceptable levels.
The permeability of the subsurface soil may limit the effectiveness of a SVE process. Air and vapor may flow through subsurface soil primarily through high permeability regions of the soil. The air and vapor may bypass low permeability regions of the soil. Air and vapor bypassing of low permeability regions may allow large amounts of contaminants to remain in the soil after a SVE process has treated the soil. Reduced air permeability due to water retention, stratified soil layers, and heterogeneities within the soil may cause regions of high and low permeability within subsurface soil.
Reduced air permeability due to water retention may inhibit contact of the flowing air with the contaminants in the soil. A partial solution to the problem of water retention is to dewater the soil. The soil may be dewatered by lowering the water table and/or by using a vacuum dewatering technique. These methods may not be effective methods of opening the pores of the soil to admit airflow. Capillary forces may inhibit removal of water from the soil when the water table is lowered. Lowering the water table may result in moist soil. Air conductivity through moist soil is limited.
A vacuum dewatering technique may have practical limitations. The vacuum generated during a vacuum dewatering technique may diminish rapidly with distance from the dewatering wells. The use of a vacuum dewatering technique may not result in a significant improvement to the soil water retention problem. The use of a vacuum dewatering technique may result in the formation of preferential passageways for air conductivity located adjacent to the dewatering wells.
Many types of soil are characterized by horizontal layering with alternating layers of high and low permeability. A common example of a layered type of soil is lacustrine sediments. Thin beds of alternating silty and sandy layers characterize lacustrine sediments. If an SVE well intercepts several such layers, nearly all of the induced airflow occurs within the sandy layers and bypasses the silty layers.
Heterogeneities may be present in subsurface soil. Air and vapor may preferentially flow through certain regions of heterogeneous soil. Air and vapor may be impeded from flowing through other regions of heterogeneous soil. For example, air and vapor tend to flow preferentially through voids in poorly compacted fill material. Air and vapor may be impeded from flowing through overly compacted fill material. Buried debris within fill material may also impede the flow of air and vapor through subsurface soil.
In situ thermal desorption (ISTD) may be used to increase the effectiveness of a SVE process. An ISTD soil remediation process involves in situ heating of the contaminated soil to raise the temperature of the soil while simultaneously removing offgas by vacuum. In situ heating may be preferred over convective heating by the introducing of a hot fluid (such as steam) into the soil because thermal conduction through soil is very uniform as compared to mass transfer through soil. Thermal conductivity of an average soil may vary by a factor of about two throughout the soil. Fluid flow conductivity of an average soil may vary by a factor of 108 throughout the soil.
Soil may be heated by radiant heating in combination with thermal conduction, by radiant by radio frequency heating, or by electrical formation conduction heating. Conductive heating may be a preferred method of heating the soil because conductive heating is not limited by the amount of water present in the soil. For soil contamination within about 2 feet of the soil surface, thermal blankets may apply conductive heat to the soil. For deeper soil contamination, heaters placed in wells may apply conductive heat to the soil. Coincident or separate source vacuum may be applied to remove vapors from the soil. U.S. Pat. No. 4,984,594 issued to Vinegar et al, which is incorporated by reference as if fully set forth herein, describes an ISTD process for soil remediation of low depth soil contamination. U.S. Pat. No. 5,318,116 issued to Vinegar et al., which is corporated by reference as if fully set forth herein, describes an ISTD process for eating contaminated subsurface soil with conductive heating.
A conductive heat ISTD soil remediation process may have several advantages ver a simple soil vapor extraction system. The heat added to the contaminated soil may aise the temperature of the soil above the vaporization temperatures of the soil ontaminants. If the soil temperature exceeds the vaporization temperature of a soil ontaminant, the contaminant will become a vapor. The vacuum may be able to draw the vaporized contaminant out of the soil. Even heating the soil to a temperature below the vaporization temperature of the contaminants may have beneficial effects. Increasing the soil temperature will increase the vapor pressure of the contaminants in the soil and allow an air stream to remove a greater portion of the contaminants from the soil than is possible at lower soil temperatures.
Most soil formations include a large amount of liquid water as compared to contaminants. Raising the temperature of the soil to the vaporization temperature of the water will boil the water. The resulting water vapor may volatize contaminants within the soil by steam distillation. An applied vacuum may then remove the volatized contaminants and water vapor from the soil. Steam distillation within the soil may result in the removal of medium and high boiling point contaminants from the soil.
In addition to allowing greater removal of contaminants from the soil, the increased heat of the soil may result in the destruction of contaminants in situ. The presence of an oxidizer, such as air, may result in the oxidation of the contaminants that pass through soil that is heated to high temperatures. Contaminants within the soil may be altered by pyrolysis to form volatile compounds that are removed from the soil by the vacuum.
Heating the subsurface soil may result in an increase in the permeability of the soil. A visible indication of the increase in permeability of soil may be seen in the surface of dry lake beds. As a lake bed dries, the soil forms a polygonal network of wide cracks. In subsurface soil, the creation of a network of cracks may result in enhanced vacuum driven transport within the soil. Laboratory measurements also indicate that the microscopic permeability of a dry mud is substantially greater than the permeability of the original mud. The macroscopic and microscopic increase in permeability of dried soil allows an ISTD soil remediation process to be applied to low permeability clays and silts that are not amenable to standard soil vapor extraction processes.
A typical ISTD soil remediation process may include four major components. The components may be heaters, off-gas collection piping, an off-gas treatment system, and instrumentation and power control systems.
For shallow contaminated soil, the heat may be applied to the soil by a heating blanket placed on top of the soil. Shallow contaminated soil includes soil contamination that does not extend below a depth of about 3 feet. For deeper contaminated soil, heat may be applied to the soil by heater wells.
The heat may be applied by a combination of radiant transfer and heat conduction. The heater element radiantly heats a casing, and the casing conductively heats the soil. The heating element of a heater well may be constructed from two NICHROME(copyright) wire loops. Interlocking ceramic beads may be positioned on the wire loops. The heating element may be supported on either side of a 310 stainless steel strip by small stainless steel bolts. The strip may be suspended from a carbon steel top hat inside a 3.5-inch stainless steel casing. The casing may be sanded into a 6-inch augered hole. The casing may include a welded top flange that seals to a silicone rubber vapor barrier placed on top of the soil. Four thermocouples may be attached to the NICHROME(copyright) heating element for temperature control. A heater well may cost approximately $180 per foot to produce. The heater well may require an installation time of about 6 man hours. control. A heater well may cost approximately $180 per foot to produce. The heater well may require an installation time of about 6 man hours.
In addition to the components of a heater well, a heater/suction well includes an outer 4.5-inch stainless steel screened liner and a flange above the surface flange. The additional flange connects to a vacuum manifold. A heater/suction well may cost about $240 per foot to produce. The heater/suction well may require an installation time of about 8 man hours.
A ratio of heater wells to heater/suction wells may be used during an ISTD soil remediation process. For example, an alternating pattern of heater wells and heater/suction wells may be used in a soil remediation system. Alternately, an ISTD soil remediation process may use only heater/suction wells. After remediation is complete, the wells may be pulled out of the ground with a crane. The holes may then be sealed by grouting to the surface. Often the condition of the wells after removal is poor. The wells may be corroded and/or bent. Extensive rework may be required to bring a well to a condition where it can be used again in another ISTD soil remediation process.
The off-gas collection piping may connect an array of suction wells to an off-gas treatment facility. The off-gas collection piping may include a plurality of metal, interconnected pipes. The interconnected piping may be flanged piping that requires careful alignment during installation. A crane may be used to lift and position the piping. The piping may be insulated piping that includes internal electric heaters. The insulation and the heaters prevent condensation of the vapor in the piping. The internal electric heaters require extra power supplies, wiring, and control units. Setting up the vapor collection piping constitutes a large part of the field installation cost of an ISTD soil remediation process.
A high soil temperature may destroy most of the soil contaminants before the contaminants are drawn to the surface facilities. A flameless thermal oxidizer may treat remaining contaminants within the off-gas stream. One commercial ISTD soil remediation system uses an 1800 scfm regenerative thermal oxidizer manufactured by Thermatrix Inc. of San Jose, Calif. The Thermatrix 1800 thermal oxidizer utilizes a ceramic media matrix to establish a stable and efficient reaction zone with an operating temperature range of 1800-1900xc2x0 F. The Thermatrix 1800 includes about 65,000 pounds of ceramic matrix that has a high thermal inertia. A saddle type geometry of the ceramic matrix promotes efficient mixing. The Thermatrix 1800 thermal oxidizer has a guaranteed destruction efficiency for chlorinated organic compounds of 99.99+%.
During initial startup, the thermal oxidizer may be preheated with a gas burner until a desired temperature profile is created. The burner is then turned off and the temperature profile inside the thermal oxidizer is maintained by addition of fuel (propane) that is mixed with air at ambient temperature. Once a stable profile is obtained, the vapor stream is allowed to enter the oxidizer. Fuel may be added or withheld from the thermal oxidizer to maintain a substantially stable temperature profile within the thermal oxidizer. Gases leaving the thermal oxidizer may be cooled in a heat exchanger. The gases may then be passed through a carbon absorption bed for backup and polishing.
Thermal oxidizers are costly to purchase, set up, and operate. The capital expense of a vapor treatment system described above is very high (more than one million dollars). Thermal oxidizers may be large and heavy units that are expensive to mobilize. For example, the Thermatrix 1800 thermal oxidizer has an on-site footprint of about 52 feet by 8 feet. The unit has 65,000 pounds of ceramic saddles. It must be transported to the site on a separate double-drop trailer. The transportation cost to and from a soil remediation site may be $70,000 or more. A thermal oxidizer requires continuous manned operation. The thermal oxidizer unit is the principal reason for manned operation of an ISTD soil remediation process.
An ISTD soil remediation process may require a large amount of computerized instrumentation for thermal well control and temperature monitoring. A well controller may be used to control a pair of thermal wells. Each well controller may monitor heater thermocouples and control power applied to a pair of thermal wells. The well controllers may be electrically connected to a central computer over a field wide data link. Each well controller may cost about $
800. Thermocouples and control wiring for the thermal wells are extensive and laborious to install, connect, and troubleshoot. Thermocouples may be driven into the soil at various locations in a region undergoing an ISTD soil remediation process to allow for temperature monitoring. The thermocouples may be polled by selected well controllers.
Well controllers enable the heater wells to apply heat to the soil at a higher rate than a steady state heat injection rate. Although a high rate can be applied at the beginning of the remediation process, the well controllers must lower the heating rate to prevent metallurgical damage to the heater wells. Thus, there may only be a small net acceleration of the heating process due to heating rate control. Moreover, the well controllers increase the chance of heater failure because they are controlling temperature at a single thermocouple location. If the thermocouple is not located at the hottest portion of a heater, the hottest portion of the heater may be maintained at an excessively hot temperature that could cause the heating element to fail.
The on-site equipment may include three trailers. The three trailers may be a process trailer, a control trailer, and an electrical trailer. The process trailer, which may contain the thermal oxidizer, heat exchangers, carbon beds and a vacuum source, may occupy approximately an 8-foot by 52-foot area. The control trailer, which contains all of the instrumentation and programming for the ISTD soil remediation system, may occupy approximately an 8-foot by 48-foot area. The electrical trailer, which provides power to the system, may occupy approximately an 8.5-foot by 48-foot area.
An ISTD soil remediation process may be used to treat a region of contaminated soil. Conductive heat may be applied to the soil by a plurality of strip heaters. For low depth soil contamination, the strip heaters may be placed in trenches within the contaminated soil. For deeper soil contamination, the strip heaters may be vertically positioned in heater wells, or in combined heater and suction wells spaced throughout the contaminated soil. Vacuum sources that are coincident to or separate from the strip heaters may be applied to the soil to remove off-gas from the soil.
A strip heater may include a heater section, transition sections, and cold pins. The heater section may be formed of a high temperature, chemical resistant metal. The heater section dissipates heat when the strip heater is connected to a power source. The metal that forms the heater section may be, but is not limited to, stainless steel, INCOLOY(copyright), or NICHROME(copyright). The specific metal used to form the heater section of a strip heater may be chosen based on cost, the operative temperature of the soil remediation process, the electrical properties of the metal, the physical properties of the metal, and the chemical resistance properties of the metal.
A heater section may have a large cross section area as compared to a cross sectional area of a conventional heater element. The large cross sectional area of the heater section may result in a smaller electrical resistance for the strip heater as compared to conventional heaters of equivalent length. The smaller electrical resistance allows several strip heaters to be connected in series. The ability to connect several strip heaters in series greatly simplifies the wiring requirements for an ISTD soil remediation system. The large cross sectional area of the heater section also allows a large contact area between the heater section and material placed adjacent to the heater section. The large contact area may promote dissipation of heat produced in the strip heater into surrounding soil. The heat is applied to the soil by conduction. Compared to conventional radiant heating, a heater strip may operate at a lower temperature for the same power input. Avoiding radiant energy transfer improves the reliability of the heating system.
A heater section of a strip heater may be formed with a rectangular cross sectional shape. For example, the heater section may be a 1-inch by xe2x85x9-inch strip of stainless steel. A heater strip may be 40-feet or more in length. Strip heaters having other cross sectional shapes may also be used. A strip heater may be formed with a variable cross sectional area so that greater heat dissipation occurs at certain portions of the strip heater (sections having a smaller cross sectional area) than at other portions of the strip heater. A local high heat dissipation section of a strip may be positioned adjacent to soil that requires extra heat dissipation, such as wet soil or the top and bottom sections to counteract heat loss. A strip heater may be formed with sections that have a large cross sectional area. A large cross sectional area section of a strip heater may be placed adjacent to an impermeable section of soil that does not need to be heated by the strip heater. The cross sectional area of sections of a strip heater may be less at the top and bottom of the heater strip so that the strip heater diffuses more energy at the top and bottom of the strip heater.
Transition sections may be welded to each end of a heater section of a strip heater. Pins may be welded to the transition sections. For example, the transition sections may be 6-inch long strips of 1-inch by xc2xd-inch stainless steel that are welded to the ends of a 1-inch by xe2x85x9-inch 20-foot long heater section. The pins may be xe2x85x9c-inch nickel pins. The pins may extend above the soil surface when the strip heater is inserted into the soil. A mechanical Kerney lug may be used to splice the nickel pins to copper cable. The copper cable may be electrically coupled to a power source, such as a transformer. Long nickel strips may be attached to a heater section to form long unheated sections of a strip heater. Long unheated sections of a strip heater may be needed for deep soil contamination that is not near the soil surface.
A strip heater that will be used to treat deep soil contamination may be bent into a U shape. The strip heater may be placed into an augered hole. The hole may be packed with sand, gravel, or with larger sized fill material. The fill material may push legs of the strip heater against a wall of the hole. Larger sized fill material may promote off-gas flow through the fill material. The fill material may acts as a thermal transfer agent between the strip heater and the soil. The fill material may include catalyst material, such as alumina, that enhances the thermal breakdown of contaminants. A suction well may be formed by inserting a perforated casing between legs of the strip heater. Attaching the perforated casing to a vacuum source allows vacuum to remove vapor from the soil as off-gas. Positioning the casing between legs of a U-shaped strip heater allows the off-gas to pass through a high temperature zone before being removed from the soil. Passing the off-gas through the high temperature zone may result in the thermal degradation of contaminants within the off-gas.
As an alternative to placing a strip heater in an augered hole, the strip heater may be driven into the soil. A drive rod may be positioned at the center of a strip heater. The drive rod may then be pounded into the soil. When the end of the strip heater is at the correct depth, the drive rod may be withdrawn. The drive rod does not need to be a continuous rod. The drive rod may be made of threaded sections that are assembled together as the drive rod is pounded deeper into the soil. A geoprobe or a cone penetrometer rig may be used to drive the heater element into the soil. Also, a sonic rig could be used to vibrate a strip heater to a desired depth. The area between the legs of the strip heater may be filled with fill material and/or a perforated casing. The perforated casing may be attached to a vacuum source to form a suction well. The fill material may include catalyst material that enhances thermal breakdown of contaminants.
Driving or vibrating a heater strip into the soil may eliminate problems associated with disposing of cuttings formed during the formation of an augered hole. Avoidance of the production of cuttings may be particularly advantageous at extremely toxic or radioactive sites. Also, driving or vibrating a strip heater into the soil advantageously places a portion of the strip heater in direct contact with the soil to be heated.
Strip heaters may be placed horizontally in contaminated soil. Horizontally oriented strip heaters may be especially useful for treating soil contamination that extends less than about 4 feet under the soil surface. Horizontally oriented strip heaters may be placed in trenches. The trenches may be formed in the contaminated soil by a trenching machine. The horizontally oriented strip heaters may be covered with the cuttings made during the formation of the trenches. The cuttings may be tamped down on top of the strip heaters. Horizontal strip heaters may be less expensive to install than are vertical strip heaters. Trenching costs are generally less than drilling costs. Also, horizontally positioned strip heaters may be very long. Rows of strip heaters may be separated by distances equal to about twice the insertion depth of the strip heaters into the soil.
The heater section of a strip heater and the power source are designed to supply heat input into the soil that is greater than the heat input that the soil can absorb, but not enough to overheat the strip heater. An average soil may be able to absorb about 300 W/ft. A strip heater may be designed to have a maximum heat input of about 600 W/ft. The temperature that a strip heater attains is self-regulating. As the temperature of a strip heater increases, the resistance of the strip heater increases. The power source provides a substantially constant voltage to the strip heaters, so an increase in the resistance of a strip heater decreases the power dissipation of the strip heater. The application of a steady voltage to a series of heater strips may result in steady state power dissipation through the strip heaters. Heater sections of strip heaters may be sized to allow the strip heaters to attain temperatures up to about 2000xc2x0 F. when energized by a power source. The strip heaters may be designed to operate at about 1600xc2x0 F. A 304 stainless steel strip heater may have a resistance of about 0.08 ohms at about 1600xc2x0 F.
The strip heaters may be directly connected by copper cable to a power source. The power source may be a transformer. A group of strip heaters may be connected in series to the transformer. The strip heaters may be directly connected to the transformer without well controllers or silicon controlled rectifiers.
The simple geometry of a strip heater may allow a strip heater to be produced at a cost of about $1.8 per foot. The production cost of a strip heater may result in about a 100xc3x97cost reduction as compared to the production of a conventional heater well. The production cost for a heater strip and suction well may be about $5 per foot. The production cost of a heater strip and suction well may result in about a 50xc3x97cost reduction as compared to the production of a conventional heater/suction well. Heater strips and heater strip and suction well combinations may not require external casings like conventional heater wells and heater/suction wells.
Installation costs of a heater strip in an augered hole may be greatly reduced. A conventional heater well took approximately 6 hours of time to both install in an augered hole and connect to a power supply. A strip heater may take 10 minutes or less to install and connect to a power supply. Installation costs of installing a heater strip and suction well combination may also be greatly reduced as compared to installing a conventional heater/suction well.
A collection system may connect all of the suction wells of a soil remediation system to a treatment facility. The collection system may include hoses and a vacuum manifold. The hoses may be high temperature hoses. The hose may be, but is not limited to a high temperature rubber hose, a high temperature silicone rubber hose, or a coated rubber flexible metal hose. The system operates under vacuum; therefore, the hose needs to have structural strength that inhibits collapse of the hose. The hose may be a double walled hose or a steel reinforced hose. The vacuum manifold may be plastic piping, such as chlorinated polyvinyl chloride piping. Off-gas passing through a hose has a residence time within the hose due to the length of the hose. The residence time may be sufficiently long to allow the off-gas to cool to a temperature within the working temperature limits of the vacuum manifold piping. A hose may be from about 4-feet to over 40-feet in length.
The use of a hose and plastic piping collection system results in lower costs, simplified on-site construction, and lower mobilization costs as compared to a conventional metal piping collection system. The collection system is not insulated and heated to prevent condensation of the off-gas. This greatly reduces the cost, installation time, and operating cost of the collection system. The hose may be rolled into coils for transportation. Plastic piping may be purchased locally near the site. Hose and plastic piping are easily cut to size on-site and are connectable by solvent gluing. The need to have precise positioning of metal pipes is eliminated. Also, hose and plastic piping are lightweight and do not require machinery to lift and position during installation. For soil contaminated with chlorinated compounds, the off-gas may contain significant amounts of HCl. Unlike metal piping, hose and the plastic piping may be highly resistant to corrosion caused by the off-gas.
A treatment facility processes off-gas from the soil to substantially remove contaminants within the off-gas. A treatment facility may also provide vacuum that removes the off-gas from the soil. The treatment facility may include a condenser that separates the off-gas into a liquid stream and a vapor stream. The liquid stream and the vapor stream may be separately processed to remove contaminants. The liquid stream may be treated using a separator and an activated carbon bed. The vapor stream may be treated using an activated carbon bed or an air stripper.
The treatment facility does not require the use of a thermal oxidizer as did previous treatment facilities. Removing the thermal oxidizer from the treatment facility eliminates the large capital cost, transportation costs, and operating expenses associated with the thermal oxidizer. The elimination of the thermal oxidizer may allow the soil remediation process to be run unattended. A site supervisor may periodically check the system and perform normal maintenance functions at the site to ensure proper operation of the soil remediation system. Continuous manned operation of the in situ soil remediation process may not be required.